Conventionally, there have been a “compression ratio” and an “expansion ratio” as design factors to determine performances of an engine. The compression ratio is a ratio of the volume before and after compression at the time of compressing the air in a cylinder. The expansion ratio is a ratio of the volume before and after expansion at the time of expanding the air (combustion gas) in the cylinder. In a general engine, the compression ratio and the expansion ratio take equal values.
An engine designed in such a manner that the expansion ratio is larger than the compression ratio is known (for example, Patent Document 1). Such an engine is called a miller cycle engine and is generally capable of adjusting opening and closing timing of an intake valve. However, in order to adjust the opening and closing timing of the intake valve, a complex link mechanism and an actuator are required, and there is sometimes a case where the timing cannot be adjusted to be optimal opening and closing timing from various factors. That is, there is sometimes a case where optimal valve timing cannot be realized. Further, there is a problem that the valve timing is varied between cylinders.